Currently DEFINE has the following syntax: <!-- DEFINE $VAR = 'value' --> that works great for setting simple variables.

Problem is, it doesn't work for complex variables. It is not possible to join several string or use another template variable as value. I suggest to extend DEFINE syntax a bit to make it more powerful:

<!-- BEGIN postrow -->
post stuff
<!-- IF postrow.SOMETHING -->
<!-- DEFINE $IMPORTANT_DATA -->
some important information inside current post that you want to display after all posts, using {postrow.WHATEVER} variables available only in this loop!
<!-- ENDDEFINE -->
<!-- ENDIF -->
more post stuff
<!-- END postrow -->
{$IMPORTANT_DATA}